One in 8 females in Canada are expected to develop breast cancer in their lifetime. (iStock)
A common oral bacterium associated with periodontal disease may drive the development and spread of breast cancer, particularly in genetically susceptible patients, according to new research from the Johns Hopkins Kimmel Cancer Center.
The study, published Jan. 15 in Cell Communication and Signaling, found that Fusobacterium nucleatum, a bacterium commonly found in the oral cavity, can travel through the bloodstream and colonize breast tissue, where it triggers DNA damage and other tumour-promoting changes.
Researchers reported that the bacterium accelerated tumour growth and increased metastasis to the lungs in preclinical models.
“The key takeaway is that this oral microbe can reside in breast tissue and that there is a connection between this pathogen and breast cancer,” said Dipali Sharma, a professor of oncology and lead investigator.
Study builds on earlier oral-systemic health research
The Johns Hopkins team said its work was inspired by earlier studies that linked periodontal disease and breast cancer risk.
A prospective cohort study of 3,273 women, published in 2010, found that chronic periodontal disease — particularly when accompanied by tooth loss — was associated with a higher incidence of breast cancer over a 16-year follow-up period.
More recently, a 2025 systematic review examining studies published between 2000 and 2024 reported that periodontal disease may be associated with breast cancer development and progression, while emphasizing that a direct causal relationship has yet to be established.
Related: Harvard dental researchers explore AI for earlier head and neck cancer detection
Breast cancer in Canada
Breast cancer is the most common cancer among females living in Canada, excluding non-melanoma skin cancer. Public Health Agency of Canada says 1 in 8 females in Canada are expected to develop breast cancer in their lifetime, and 1 in 36 are expected to die from it.
Ontario Health Study material has also noted that while diagnoses in older women have declined over the past 25 years, diagnoses among women under 50 have increased.
DNA damage and aggressive tumour behaviour
Using human breast cancer cells and mouse models, the new study found that exposure to F. nucleatum:
induced DNA damage and inflammatory responses
triggered abnormal cell growth and tissue changes
activated error-prone DNA repair pathways
increased tumour cell migration, invasion and chemotherapy resistance
The researchers also reported increased expression of DNA-PKcs, a protein linked to tumour progression and aggressive cancer behaviour.
Related: US$6M in new NIH funding to find treatment for ‘growing’ oral cancer and its pain relief
Higher vulnerability in BRCA1-mutant cells
The study found especially strong effects in breast cells with BRCA1 mutations, an inherited genetic change associated with higher breast cancer risk. BRCA genes normally help repair DNA damage, but harmful mutations can weaken that repair system.
In the Johns Hopkins study, BRCA1-mutant cells showed:
greater bacterial uptake and retention
increased DNA damage over time
enhanced tumour-promoting activity
Researchers said that may be due in part to higher expression of surface molecules that make it easier for the bacterium to bind to and enter cells.
“Multiple risk factors come together, with F. nucleatum acting as an environmental factor that may cooperate with inherited BRCA1 mutations to promote breast cancer and tumour aggressiveness,” Sharma said.
Clinical implications still unclear
Researchers cautioned that more studies are needed before oral health can be considered a modifiable risk factor in breast cancer prevention or management. However, the findings add to growing evidence connecting oral health and systemic disease, particularly in higher-risk populations.
Related: Oral bacteria and fungi tied to higher pancreatic cancer risk, large cohort study finds